/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
class Solution {
    public ListNode mergeTwoLists(ListNode l1, ListNode l2) {
        if(l1 == null) {
            return l2;
        }
        if(l2 == null) {
            return l1;
        }
        ListNode head = null;
        if(l1.val < l2.val) {
            head = l1;
            l1 = l1.next;
        }else {
            head = l2;
            l2 = l2.next;
        }

        ListNode cur = head;
        while(null != l1 && null != l2) {
            if(l1.val < l2.val) {
                cur.next = l1;
                l1 = l1.next;
            }else {
                cur.next = l2;
                l2 = l2.next;
            }
            cur = cur.next;
        }
        if(null != l1) {
            cur.next = l1;
        }
        if(null != l2) {
            cur.next = l2;
        }
        return head;
    }
}